The blood coagulation cascade has received much attention. See generally C. van't Veer and K. Mann, Semin. Thromb. Hemo., 26(4): 367–372, (2000).
More particularly, the blood coagulation cascade is thought to be initiated when subendothelial tissue factor is exposed to blood flow following either the damage or activation of the endothelium. Blood vessel damage exposes blood to cells that contain a significant amount of a transmembrane protein called tissue factor (TF). Such exposure facilitates fibrin clot formation. See e.g., C. van't Veer and K. Mann, Semin. Thromb. Hemo., 26(4): 367–372, (2000).
TF is understood to be a cell surface receptor for plasma-derived coagulation factor VIIa. Binding of the serine protease factor VIIa to TF has been reported to cause a rise in catalytic efficiency of the enzyme leading to the initiation of the coagulation cascade by the activation of factors IX and X. TF is thought to be a prerequisite for the enzymatic action of factor VIIa. TF is thus an important component of normal blood coagulation.
TF forms a complex with Factor VIIa. That complex is believed to activate a fraction of the circulating zymogen factors X and IX to their respective active forms, factors Xa and IXa.
In particular, factor IXa assembles with factor VIIIa on a phospholipid surface to form an what is known as an intrinsic tenase complex. That complex is thought to produce additional factor Xa. The resulting factor Xa produced via both pathways assembles on an anionic cellular surface with factor Va into what is referred to as a prothrombinase complex. That complex activates prothrombin to α-thrombin; an important step in blood clot formation.
There is almost universal recognition that thrombin is important for the formation of a stable fibrin clot.
For example, thrombin cleaves fibrinogen and the resultant fibrin polymerizes to form the clot. Thrombin also accelerates its own generation by activation of the pro-cofactors V and VIII and by activation of blood platelets. The activated platelets expose the necessary phospholipid equivalent surface for the formation of the factor IXa-factor VIIIa and factor Xa-factor Va enzyme complexes. Moreover, thrombin stabilizes the fibrin clot against proteolytic degradation by the fibrinolytic system via activation of the thrombin-activatable fibrinolysis inhibitor (TAFI) and via activation of factor XIII. See e.g., C. van't Veer and K. Mann, Semin. Thromb. Hemo., 26(4): 367–372, (2000)).
There has been much interest in identifying and controlling bleeding disorders. See for example, C. van't Veer et al., Blood, 95(4): 1330–1335, (2000); C. van't Veer and K. Mann, Semin. Thromb. Hemo., 26(4): 367–372, (2000); C. Negrier et al., Semin. Thromb. Hemo., 26(4): 407–410, (2000); A. Shapiro, Semin. Thromb. Hemo., 26(4): 413–419, (2000); S. Schulman, Semin. Thromb. Hemo., 26(4): 421–424, (2000).
See also C. Negrier et al., Semin. Thromb. Hemo., 26(4): 407–410, (2000); A. Shapiro, Semin. Thromb. Hemo., 26(4): 413–419, (2000); S. Schulman, Semin. Thromb. Hemo., 26(4): 421–424, (2000).
Particular attention has been focused on understanding the role of factor VII in blood coagulation.
For example, most factor VII is believed to circulate as a single chain zymogen (10 nM) and a trace (˜10–100 pM) circulates in the active 2-chain form. Factor Xa, factor VIIa-TF, thrombin, factor IXa, and factor XIIa have been reported to activate factor VII. A comparison of the catalytic efficiencies of the potential physiologic factor VII activators showed that factor Xa, in association with phospholipids possesses the highest potency to activate factor VII. See e.g., C. van't Veer et al., (2000) Blood, 95(4): 1330–1335.
Specific examples of bleeding disorders are known including heritable forms such as hemophilia A and B. These disorders are believed to be impacted by deficiency of coagulation factors VIII and IX, respectively. Attempts to treat these blood disorders have involved “replacement therapy” i.e., administration of supplemental factor VIII or IX.
Other hemophilia treatment methods have involved therapy with recombinant factor VIIa. Such therapy has been reported to be effective indicating the potential for a strong factor VIIa-dependent enhancement of the thrombin generation process in vivo. There is recognition that at least part of the therapeutic effect may stem from overcoming the inhibitory effect of physiologic concentrations of zymogen factor VII on TF-dependent hemorrhagic control. See C. van't Veer and K. Mann, Semin. Thromb. Hemo., 26(4): 367–372, (2000).
There is recognition that certain membrane settings may assist procoagulant complexes. For example, certain activated aggregated platelets are thought to provide procoagulant phospholipid-equivalent surfaces upon which the complex-dependent reactions of the blood coagulation cascade are localized. See K. Mann, Thrombosis and Haemostasis; 82(2):165–174, (1999).
There have been problems practicing prior therapeutic methods for treating blood coagulation disorders with blood coagulation factors such as recombinant Factor VIIa (rFVIIa).
For example, most commercial sources of Factor VIIa are expensive. Much needed treatment can involve high consumption of the blood factor. Supraphysiological levels of recombinant factor VIIa (˜300 X normal) are often needed to treat hemophilia. Accordingly, prior treatment methods can be prohibitively expensive for some patients. See J. Ingerslev, Seminars in Thrombosis and Hemostasis; 2000, 26(4):425–432.
There have been other problems implementing prior strategies to treat blood coagulation disorders.
For example, a particular disadvantage of treatments involving recombinant FVIIa (rFVIIa) is that this factor has a short half life. Moreover, many rFVIIa treatment protocols require multiple injections (frequently>100). Many of the protocols require treatment over an extended period of time, especially after surgical procedures. Patient inconvenience and discomfort has been substantial.
There have been attempts to address these and related shortcomings, particularly with respect to treatment regimens involving rFVIIa. For example, continuous rFVIIa infusion therapies have been adopted. However, the costs of such treatment are often expensive. See S. Schulman, Semin. Thromb. Hemo., 26(4): 421–424, (2000).
Recombinant factor VIIa has been extensively used for the treatment of hemophilia A and B patients with inhibitors, although the exact mechanism by which this enzyme, used at supraphysiological concentrations, restores normal hemostasis is not secure. Additionally, the lack of correlation between the in vivo factor VIIa levels during the treatment and the efficacy of the treatment, makes the outcome of treatment somewhat unpredictable. See Santagostino E, et al., Relationship between factor VII activity and clinical efficacy of recombinant factor VIIa given by continuous infusion to patients with factor VIII inhibitors. Thromb Haemost. 2001;86:954–958.
Other drawbacks to conventional therapies used to treat blood coagulation disorders including danger from infectious viruses including those capable of disabling the immune system (e.g., HIV). Methods have developed to ensure blood factor purity. However, complete safety has been difficult to ensure.
Another major drawback to conventional therapies used to treat blood coagulation disorders is that some patients develop high-titer, inhibitory antibodies to blood coagulation factors. Therefore, such patients can no longer be treated with conventional blood coagulation factor replacement therapy and alternative methods for treatment need to be found.
It would also be desirable to have compositions for preventing or treating blood clotting disorders that are cost effective and require relatively short treatment times for optimal results. It would be further desirable to have compositions which include at least one blood coagulation factor, preferably activated, and phospholipid to enhance function of the composition.